Millimeter wave radio-relay communication systems are designed for providing high throughput point-to-point and point-to-multipoint communication over the distances up to several kilometers in line-of-sight conditions. Such systems are widely used in different transport networks for various applications, where the most advanced are backhaul networks between cellular communication base stations.
Current radio-relay systems use different radio-frequency bands from 2 to 100 GHz. With growing demands for the transmission data rate, use of high-frequency bands is becoming more advantageous. With higher carrier frequency, greater transmission data rate is usually achieved by using broader band connectivity.
However, free space propagation losses grow with the increase of a carrier frequency value. To compensate the losses, aperture antennas (which have antenna size much greater than operating wavelength) with high directivity and, consequently, narrow radiation pattern (RP) beam are used.
Further, narrow antenna beam allows every radio-relay communication link to operate independently from other communication links and ensures interference-free conditions for any devices located proximately due to significant localization of radiation in space. Thus, for example, the Russian State Radio Frequency Commission requirements as of 15 Jul. 2010, No. 10-07-04-1 originally prescribed that antenna pattern beamwidth for the radio-relay system operating in the wavebands 71-76/81-86 GHz shall not exceed 1 degree. However, at present the document No. 10-07-04-1 was modified (modifications were approved by the Russian State Radio Frequency Commission decision No. 14-27-07 approved 13 Oct. 2014) and accordingly antenna pattern beamwidth shall not exceed 2.5 degree. The corresponding maximum allowed beamwidth in 71-76/81-86 GHz bands in USA is 1.2 degree as it is set by Federal Communication Commission, Title 47 of the Code of Federal Regulations, Part 101.115.
However, narrow-beam antennas may be easily misaligned and may disrupt communication even with slight changes in orientation of the radio-relay communication system caused by accidental sways of its mounting structures, for example, due to vibrations, wind, or heating of the mounting structures. To provide quick automatic and unmanned beam adjustment in the angle ranges of several beam widths, aperture antenna devices with electronic beam scanning are introduced and becoming more commonly used in different fields of radio communications, including different radar applications, local communication systems, and radio-relay communication systems. Beam-scanning antennas also often provoke a redesign of the transceiver system.
Electronic beam adjustment allows the main beam direction of the antenna pattern to be quickly automatically adapted to compensate changing in orientation of the radio-relay communication system. Additionally, beam electronic adjustment significantly simplifies the mechanical positioning system for allowing precise antenna adjustment.
Electronic beam scanning can be implemented in several ways. For large aperture antennas, the most efficient way of scanning is switching over a number of beam directions. Such switching is accomplished by means of high-frequency commutation circuit connected to the feed antenna array of a given aperture antenna.
However, one disadvantage of this switching approach is increased losses in the high-frequency signal commutation circuit. Such commutation circuit generally comprises at least one semiconductor switch which cannot be realized without any losses due to technological limitations. Apparently, these losses will grow with the increase of the operating frequency in the circuit that can be shown by analysis of commercially available switches of various frequency bands. Existing N-position switches of a frequency band over 60 GHz cause losses of about 0.7÷1.5*N dB. For example, losses in a 4-position switch TGS4306-FC produced by Triquint Semiconductors are over 3 dB (that means that switching causes half the power loss). Further, one should also consider losses (about 1 dB) caused by installation of the switch into a switching circuit (for example, on a printed board).
The following is a description of known antennas with the electronic beam scanning, as well as radio-relay communication systems and radars using beam-scanning antennas of various configurations.
Point-to-Point Radio-Relay Communication Systems with Electronic Beam Scanning
PCT/RU2011/000814 “SYSTEM AND METHOD OF RELAY COMMUNICATION WITH ELECTRONIC BEAM ADJUSTMENT” discloses a point-to-point radio-relay communication system with electronic beam steering. The system comprises two line-of-sight transceiver units. Each of transceiver units comprises an antenna providing electronic switching of beam direction and a control module configured for implementation of the antenna beam control algorithm based on input of the system's service information.
Further, there are a number of different configurations of beam-scanning antennas known. Thus, U.S. Pat. No. 7,834,803 discloses the Cassegrain antenna with electronic beam steering. This antenna comprises a Cassegrain antenna (or any other type of antennas with space-apart feeding radiators) and an array of horn antennas performing the function of primary feed antenna elements. Such antenna allows electronic steering of the beam in various radar applications.
PCT/RU2011/000371 discloses another beam steering antenna. The antenna is an integrated lens antenna with an array of primary antenna elements placed on a plane surface of the lens generating a narrow beam when each one of antenna elements is excited and all other are inactive. Placement of antenna elements on the plane surface of the dielectric lens is what distinguishes integrated lens antennas from other types of lens antennas, such as horn lens antennas, Fresnel lens, thin (as compared with focal distance) lens with separately standing primary antenna elements. Such location of antenna elements reduces electric length of the wave when it is radiated into the lens: the greater the dielectric permittivity, the shorter the distance. This provides miniaturization of the antenna elements and their placement at small distances from each other. Thus, the required space of the antenna array is made considerably smaller than for other types of antennas with antenna elements and the main focusing device (mirror or lens) spaced apart from each other.
On the other hand, close placement of antenna elements ensures small angular distance between the directions of main antenna pattern beams during scanning. Based on this, development of scanning integrated lens antennas becomes possible, thus ensuring sufficiently large overlap of beams during scanning and, consequently, enabling scanning in a certain continuous angle range exceeding antenna beamwidth. This advantage of integrated lens antennas is particularly significant for the radio-relay communication applications.
Point-to-Multipoint Radio-Relay Communication Systems with a Plurality of Antennas
U.S. Pat. No. 7,844,217 “POINT-TO-MULTIPOINT COMMUNICATION TERMINAL HAVING A SINGLE RF CHAIN” discloses a radio-relay communication system comprising two directional antennas with fixed beams to communicate with spaced-apart remote terminals (see FIG. 1). The system comprises only one transmitter and one receiver, while an antenna for transmission and reception of signal is selected with a high-frequency switching circuit. It is evident that the system can only provide communication in time division duplexing mode. Additionally, transmission of the signal to two remote terminals is to be carried out during different time intervals, which reduces the total data rate of communication between the system and either of the remote terminals. At the same time, the system's modem shall comprise a selection unit of information stream for signal processing from/for either of the remote terminals, synchronized with the high-frequency switching circuit.
A disadvantage of the system is also high losses in the switching circuit operating in the millimeter wave band. Furthermore, the system does not provide frequency division of reception and transmission to enable reception and transmission simultaneously in different frequency bands.
Automotive Radars of Millimeter Wave Band
The design of automotive radars of millimeter wave band is largely similar to the design of radio-relay systems with a beam-scanning antenna. U.S. Pat. No. 6,034,641 “ANTENNA DEVICE” discloses the automotive radar device. A general diagram of this device is shown in FIG. 2.
The automotive radar comprises a plurality of independent radar elements, each comprising RF reception and transmission modules, switching circuits of received and transmitted signals and an array of primary antenna elements which are designed to provide electronic scanning in the antenna system. The antenna system can be based on the lens antenna, reflector antenna or other type of aperture antennas.
Disadvantageously, usage of a high-frequency signal switching circuit leads to additional losses in the radio frequency front-end. Such additional losses result in smaller range of coverage of the radar or of the radio communications system, which might in some cases makes the use of electronic beam scanning systems inefficient or impossible.
The most obvious way to reduce losses in a high-frequency switching circuit is to develop a more effective millimeter wave band switches. However, this approach appears to be rather hard to implement and fails to ensure zero losses due to insufficient development of semiconductor technologies at present.
U.S. Pat. No. 5,486,832 “RF SENSOR AND RADAR FOR AUTOMOTIVE SPEED AND COLLISION AVOIDANCE APPLICATIONS” discloses another solution applied to automotive millimeter wave band radars. The diagram of this solution is shown in FIG. 3. The device uses an antenna providing electronic beam scanning by means of switching between primary antenna array elements. The switching circuit of this device is implemented at baseband frequency and uses a set of received signal mixers, each connected to one primary antenna element.
However, the automotive radar cannot be used in radio relay systems for a number of reasons. First, this radar can scan only with a receiving beam, as antenna elements are only connected to receiving mixers. Signal radiation is accomplished by a separate antenna with a broad radiation pattern. The radio-relay system requires both signal reception and signal transmission, which is conceivable if, instead of receiving mixers, more complex radiofrequency units are used, which are capable of processing the received signal and generating transmission signal using one of the known radio transmission duplex modes. Further, two signal distribution (switching) units are required. Secondly, in the radar under consideration, sequential switching across all antenna elements to be processed in the CPU is provided, which is determined by specific features of radars. That is why, the input of the beam selection unit is connected only to the reference signal generator, based on which sequential switching of beam position takes place. In the radio-relay system, the beam position control module shall have expanded functionality, including ability to switch the beam randomly using the built-in algorithms and service information received from the system's modem. This is predetermined by the random nature of twists and sways effects in the mounting structures exposed to vibrations, wind, heating of the components, etc., which effects are sought to be compensated by means of beam-scanning antennas in radio-relay systems.
The prototype for the present invention is a point-to-point radio-relay communication system with electronic beam scanning disclosed in PCT/RU2011/000814. However, usage of the beam-scanning antennas in such a radio-relay communication system requires a switching circuit distributing the signal from the transceiver to one of primary antenna elements.
Generally, the prototype system comprises (FIG. 4):
a beam-scanning antenna with at least two primary antenna elements,
a radio-frequency transceiver unit,
a received signal distribution unit configured to distribute a received signal and connected to the radio-frequency transceiver unit,
a transmitted signal distribution unit configured to distribute a transmitted signal and connected to the radio-frequency transceiver unit,
a digital-to-analog and analog-to-digital converter units,
a modem comprising a modem reception part and a modem transmission part and
an antenna beam control unit configured to control a beam direction and connected to the modem reception part and the modem transmission part via supervisory channels, the unit being further connected to the received signal distribution unit and the transmitted signal distribution unit via beam control channels, thus enabling supply of beam control signals to them.
The beam-scanning antenna can provide electronic beam scanning by means of switching between primary antenna elements accomplished by the switching circuit which switches the transceiver signal onto one of the antenna elements. The radio-frequency transceiver unit comprises a receiver, a transmitter and a signal duplexer. The received signal distribution unit and the transmitted signal distribution unit are formed as a single distribution unit of signal from the radio-frequency transceiver unit. The modem performs digital processing of signals, while the antenna beam control unit generates control signals for the signal distribution unit based on service data provided by the modem.
FIG. 4 shows that the switching circuit is to perform switching of high-frequency signal generated on the carrier frequency. Practically, these frequencies are in the range of 30-100 GHz. Currently, switching of such a high frequency signal cannot be done without loss. As said above, known millimeter wave band switches with one input and N outputs have losses of about 0.7÷1.5*N dB, and, thus, a four-position switch has losses of approximately 3.5-4 dB. These losses are doubled at the radio-relay system's mate side, which causes a considerable loss of signal strength and, consequently, shortens the maximum signal coverage more than two times.
Thus, for efficient and feasible implementation of millimeter wave band beam-scanning antennas and for larger coverage of radio communications systems with beam-scanning antennas operating in transceiver duplex mode, it is necessary to reduce losses in the switching circuit.